Damaged lining of metallurgical units is normally patched by using what is known as wet and semidry guniting techniques which consist in applying a moistened powdered refractory mass to a damaged area of the lining. Needless to say, the metallurgical unit is allowed to cool before patching. This type of lining repair takes much time, and the life of the lining is quite limited; the reason for this lies in the fact that the heat resistance of the refractory mass is reduced by additives which serve to ensure a desired degree of plasticity of the mass in the cold state. For example, a magnesite- or dolomite-based refractory mass used for the patching of damaged converter lining has to contain silicate-based additives. Such a patching material differs in composition from the lining material; additives contained in the patching material react with the lining material to produce compounds whose refractoriness is lower than that of the original lining material; a patching mass of this type does not adhere well enough to the original lining and is not as resistant to the chemcial attack of slag as the original lining material.
The most effective patching technique is the flame spraying of gunite on the lining of metallurgical units. This method consists in feeding a refractory material, fuel and oxygen to the lining, burning the fuel in an oxygen flame, heating particles of the refractory material caught in the flame to a temperature at which they assume a plastic state, and applying the plastic particles of the refractory material to the linings under repair.
There are known a method and device for flame guniting of converter lining. According to this method, the refractory and fuel are both powdered materials which are fed to the lining together. A jet carrying the refractory, fuel and oxygen is directed tangentially to the surface of the converter lining (Cf. FRG Pat. No. 2,200,667, Cl. C 21 C, 5/44/, published May 13, 1976).
The tangential positioning of the flame relative to the lining surface is disadvantageous in that the patching is not good enough, since much of the refractory material is carried away as dust with the discharged combustion products. The latter, in turn, is due to incomplete combustion and also to the fact that a rarefield zone is produced in the center of the rotating dust-carrying gas jet, which zone attracts both gases and particles of the refractory material. With a tangential positioning of the flame relative to the lining surface, the distance between the ends of the nozzles and the lining is not enough to ensure complete fuel combustion; as a result, the heating is inadequate to plasticize the refractory material. The patching material thus contains refractory particles which poorly fuse with one another and with the original lining material. The poorly heated refractory material does not adhere to the lining and is carried away with the combustion products.
There is also known a method for flame spraying the lining of metallurgical units, wherein a mixture of a refractory material, fuel and oxygen is directed at a perpendicular or at an angle to the surface of the lining that needs patching. As a result, the refractory material is softened by the high-temperature flame; the additives contained in the refractory material and fuel are fused, and wet the lining surface and the particles of the refractory material in the flame. Particles of the refractory material are projected against the lining surface at an angle close to 90.degree., and the refractory adheres effectively to the lining.
The art closest to the present invention is the flame spraying method, wherein a jet of fuel and refractory material and a jet of oxygen are directed at a perpendicular to the surface of the lining in need of patching. This method is carried out with the aid of a tuyere formed with channels for separate feeding of compressed oxygen and a powdered mixture of refractory and fuel to respective nozzles extending along the tuyere wall.
The mixture of refractory and fuel is passed through a central nozzle with oxygen being fed through an annular channel between the nozzles. The mixture comprises 20 to 30 percent by weight of fuel and 70 to 80 percent by weight of magnesite (cf. the journal "Metallurg", Metallurgia Publishers, Moscow, 1977, No 12, pp. 25-26).
The latter method has the disadvantage that it does not provide for the uniform heating of all the refractory particles in the flame; furthermore, it does not ensure equal speeds of refractory particles directed at the lining surface, because the fuel combustion zone is extended along the flame; the asymmetric flow of fuel and refractory material does not mix well enough with the annular flow of oxygen; and the distribution of concentrations and temperatures is not uniform over the cross-section of the flame. With the two-phase mixture of fuel and refractory material delivered in the form of a central jet and oxygen delivered as an annular jet, the concentration of fuel and refractory material is higher in the middle of the cross-sectional area of the flame than on the flame periphery. On the contrary, the oxygen concentration is higher on the periphery of the flame than in its center. Combustion is not completed over the short period of time it takes a particle to reach the lining; as a result, part of the refractory material is not heated so that it can assume a plastic state poorly heated refractory particles are entrained as dust in the outgoing flow of combustion products. The resultant patching is not durable enough, because also, poorly heated refractory particles that reach the lining surface do not effectively fuse with one another and with the lining material, which affects the bonds between grains of the patching material. With the mixture of fuel and refractory material delivered in a flow parallel to the flow of oxygen, the amount of refractory material adhering to the lining is never higher than 60 to 70 percent of the total amount of refractory material used for the patching.